1. Field of the Invention
This invention relates to the recovery of liquid hydrocarbons from coal and is particularly concerned with an improved in situ recovery process which permits the recovery of hydrocarbon liquids in substantial quantities.
2. Description of the Prior Art
Considerable work on the underground gasification of coal has been done in the past, particularly in the Soviet Union, Great Britain, Belgium, France, Italy and the United States. The early work for the most part was directed toward the injection of air into underground passageways produced by mining operations to permit the recovery of combustible gases containing substantial quantities of hydrogen and carbon monoxide. To improve contact between the injected gas and the coal, it was proposed that coal be broken down from the walls by cutting, drilling and shooting operations and that suitable barriers be introduced within the headings or tunnels to force the gases to move through the loosely piled coal. The use of stoping, where applicable, has also been proposed as a means for providing broken coal through which the gases may be passed. The most successful operations of this type have been those carried out in steeply tilted formations where air is injected from the earth's surface through a first tunnel extending downdip in the seam, combustion takes place in a second, horizontal tunnel extending along the strike, and the combustion products are withdrawn through a third tunnel extending updip to the earth's surface. Combustion takes place along the roof of the horizontal tunnel and hence the ash and any collapsible rock fall into the bottom of the tunnel. This permits the movement of air along the face of the coal as the combustion front moves upwardly in the seam and avoids blockage of the passageways. This method is reportedly still used in the Soviet Union but has limited application because of the requirement that the coal seam be steeply tilted.
Because of the high cost of underground mining operations, there have been numerous attempts to carry out underground gasification between boreholes drilled into coal seams from the earth's surface. Coal normally has some permeability and when heated tends to shrink, crack and become more permeable. In general, however, this permeability is not sufficient to permit effective gasification between boreholes and hence some method for providing an initial passageway between the boreholes must be employed. An early proposal suggested the injection of air through a central pipe string in each of two boreholes and the recovery of combustion gases through the annulus surrounding each pipe string until a cavity had been burned out at the bottom of each hole and communication between the holes had been established. Thereafter, one borehole was used for the injection of air and the other was employed for the recovery of combustion products. Other methods which have been proposed include the use of hydraulic or pneumatic pressure to fracture the coal between boreholes, the use of electrodes between which an electric current can be passed to carbonize the coal and create a permeable channel, the use of explosives to shatter the coal between boreholes, the use of nuclear devices to create shattered zones of high permeability, the use of directional drilling to establish underground passageways between boreholes spaced some distance apart at the earth's surface, and the injection of acids or other chemicals into the coal seam to react with the coal and create zones of relatively high permeability through which gases can be subsequently passed. All of these methods are intended to permit the injection of air or oxygen, alone or in combination with steam, into the coal seam and the recovery of gases containing hydrogen and carbon monoxide in relatively high concentrations. These gases have relatively low Btu contents but can be treated for the removal of carbon dioxide and sulfur and nitrogen compounds and then employed as low grade fuel gases or upgraded by conventional methanation operations carried out at the surface. They can also be further processed for the recovery of hydrogen or for use as feedstocks to Fischer-Tropsch or similar processes.
Comparatively little work has been done on in situ processes for the recovery of liquids from coal. It has been observed that the gases produced during underground gasification operations may contain tars and some low molecular weight hydrocarbons. There have been suggestions that hydrogen and various aromatic hydrocarbons might be injected into underground seams at high temperatures and pressures to hydrogenate a portion of the coal and permit the recovery of liquid products. It has been proposed that nuclear explosives be detonated in oil shales and other formations to create cavities and permit the recovery of vaporized or liquefied hydrocarbons. In general, however, these suggestions have been speculative in nature. No process of this type which appears commercially feasible has yet been developed.